class_docstr: a immutable object holding a source code fragment,
  possibly deindenting it.
code: def __getitem__(self, key):
      if isinstance(key, int):
          return self.lines[key]
      else:
          if key.step not in (None, 1):
              raise IndexError("cannot slice a Source with a step")
          return self.__getslice__(key.start, key.stop)
  
imports: from __future__ import generators
  from bisect import bisect_right
  import sys
  import inspect, tokenize
  import py
  from types import ModuleType
label_desc: Use image node_id 3 for calling the Source obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self, self